SQL & PL/SQL

Hi guys.

I'm new to this and i've already ran into some trouble. I am required to write a transaction for college. I am using Oracle Express Edition and it is giving me an error on my variables regarding the &

Below is my code:

DECLARE
accnum transactions.account_num%type:=&accnum;
tamt transactions.trans_amt%type:=&tamt;
tcomm transactions.trans_comments%type:=&tcomm;

BEGIN

INSERT INTO TRANSACTIONS VALUES (accnum, sysdate, tamt, tcomm);

UPDATE ACCOUNT_TYPE
SET balance = balance - tamt
WHERE account_num = accnum;

COMMIT;
END;


So when I declare a variable I get an error for the &.
E.g. accnum transactions.account_num%type:=&accnum;

The error is:
ORA-06550: line 2, column 39:
PLS-00103: Encountered the symbol "&" when expecting one of the following:

( - + case mod new not null


I have searched for another way of declaring a variable that prompts the user to enter it but did not find one. This is the way it was shown in college :) and I believe nobody has experienced this error in college. So could it be something to do with Oracle Database Express Edition?

Thanks,
Lewis.